The combustion of carbon containing fuels (biomass, waste, fossil fuels such as coal, lignite, hydrocarbons, . . . ) produces CO2 and gases, such as SO2, SO3, NOx, which pollute the atmosphere and are major contributors to the greenhouse effect especially CO2. These emissions of CO2 are concentrated in four main sectors: power generation, industrial processes, transportation, and residential and commercial buildings. The main application of CO2 capture is likely to be in power generation and large energy consuming industries, particularly cement, iron and steel and chemical production and oil refining. Capturing CO2 directly from small and mobile sources in the transportation and domestic and commercial buildings sectors is expected to be significantly more difficult and expensive. Most of the emissions of CO2 to the atmosphere from the electricity generation and industrial sectors are currently in the form of flue gas from combustion, in which the CO2 concentration is typically 4-14% by volume, although CO2 is produced at high concentrations by a few industrial processes. In principle, flue gas could be stored, to avoid emissions of CO2 to the atmosphere it would have to be compressed to a pressure of typically more than 100 bar abs and this would consume an excessive amount of energy. Also, the high volume of the flue gas would mean that storage reservoirs would be filled quickly. For these reasons it is preferable to produce relatively high purity stream of CO2 for transport and storage; this process is called CO2 capture. This carbon dioxide could be used for enhanced oil recovery or just injected in depleted gas and oil fields or in aquifers.
The present invention is based on application to the power generation sector. Nevertheless, it could also be applied to flue gases coming from other industrial processes with a relatively high purity, above 50% by volume (dry base).
There are three main techniques for capture of CO2 in power plants:                Post-combustion: the flue gas from a power station is scrubbed with a chemical solvent such as an aqueous solution of amines which will remove the CO2 by absorption;        Pre-combustion: the fuel together with oxygen is sent to a gasifier where a synthesis gas (main component of the mixture: H2, CO and CO2) is produced. CO is then shifted to H2 and CO2 (CO+H2O< >CO2+H2) and CO2 is scrubbed by a physical or chemical solvent. A mixture containing essentially H2 and N2 is sent to a gas turbine where it is burnt.        Oxycombustion: in order to increase the carbon dioxide content in the flue gas, the fuel is burnt with a mixture of mainly carbon dioxide and oxygen instead of air. This mixture of oxygen and carbon dioxide is obtained by recycling part of the flue gas rich in carbon dioxide and mixing it with oxygen (typically at 95% purity) coming from a cryogenic air separation unit. The flue gas is then purified in order to remove components like water and oxygen and compressed to a pressure between 100 and 200 bar abs in order to be injected underground (see FIG. 1). It should be noted that the recycling of flue gases would not be necessary with high temperature materials for the boiler. However, they do not exist at the time of invention. The recycling of flue gases is not mandatory for the invention disclosed here in.        
EP-A-0503910 describes a process for the recovery of carbon dioxide and other acid gases from flue gases coming from a power plant using the oxycombustion technique
A more recent document on the same subject is “Oxy-Combustion Processes for CO2 Capture from Power Plant” IEA Report No. 2005/9, September 2005, Process Flow Diagrams 6, p. 1, and 11, p. 1.
The purpose of this invention is to improve the solution proposed in this patent both in term of specific energy and/or carbon dioxide recovery and/or carbon dioxide product purity.